1. Field of the Invention
The present invention relates to a method of manufacturing extremely precise glass optical elements such as lenses and prism and to glass optical element preform molded by reheating processes.
2. Description of the Prior Art
Recently, glass optical elements have significantly tended to have an aspherical surface to allow manufacturers to simultaneously achieve simple construction and light weight of lenses for use with optical apparatuses. Since any of the conventional polishing methods for manufacturing optical lenses involves difficulty in the processing and mass production of these aspherical lenses, a molding process using a mold is considered to be quite promising as was disclosed in the U.S. Pat. No. 3,900,328, U.S. Pat. No. 4,139,677, and the U.S. Pat. No. 4,168,961, for example. The proposed molding process using mold thermally molds a roughly shaped optical-glass material by applying a non-spherical mold complete with the desired surface quality and surface accuracy or thermally molds a gob of optical glass.
When manufacturing glass optical elements like aspherical lenses and prisms by molding via recurrent thermal processes, it is essential for the manufacturers to use the molded glass optical preform perfectly free from minimal concave and convex surfaces or scratches on the surface, i.e., the molded glass optical preform must have a perfect mirror surface having a maximum of 0.005 microns of surface roughness (RMS) for example. As was cited in U.S. Pat. No. 4,139,677 and the Japanese Laid-Open Patent Publication No. 59-116137 of 1984 for example, melted glass was thus far directly molded by a mold or ground and polished before eventually yielding a conventional optical glass preform. As a result, a variety of complicated steps were needed for grinding and polishing melted glass to result in high cost. On the other hand, there is a conventional art which directly receives melted glass by initially coating with a carbon layer of a parting agent by generating vapor-phase reaction on the mold like molding plate or pressurized mold before eventually processing glass heated to high temperature. Typically, this art was proposed by the Japanese Laid-Open Patent Publication No. 54-60312 of 1979 for example. However, since this prior art merely produced porous and uneven film of carbon layer of parting agent, a variety of problems occurred. For example, residual parting agent easily adhered to the glass surface. As a result, glass surface could not achieve perfect mirror surface having a maximum of 0.005 microns (RMS) of surface roughness. Furthermore, very fine concave and convex surfaces were also generated on the glass surface. In addition, as a result of oxidation and wear of the carbon layer, life of the mold itself became short.